Four-wave mixing is a process in an optical medium where three coherent optical waves interact with one another through nonlinear coupling to produce a fourth coherent signal wave. The third-order nonlinear susceptibility of the medium primarily contributes to such nonlinear coupling. The signal wave includes information on optically-excited atoms or molecules present in the medium where the three input optical waves intersect and hence can be collected to extract information about the medium. For example, the signal strength of the signal wave is associated with the population of atoms or molecules and the spectral characteristics of the signal wave can be analyzed to reveal the structure of the atoms or molecules of interest. The coherent characteristics of the four-wave mixing signal beam have a number of advantages, including a laser-like signal beam, efficient signal collection, excellent spatial resolution, and sub-Doppler spectral resolution. Hence, four-wave mixing has been widely used as a highly sensitive tool in spectroscopic measurements and many other applications requiring detection of a minute amount of a substance.
One commonly-used four-wave mixing process is the backward-scattering degenerate four-wave mixing where three input beams (two pump beams and one probe beam) are at a common frequency. The nonlinear wave mixing produces a fourth signal beam at the same common frequency as the input beams. In a two-dimensional configuration, the wave vectors of the three input beams are in the same plane when mixed inside the medium. Due to conservation of momentum, the wave vector of the generated signal beam is also in the same plane. When two pump beams counter propagate and the probe beam intersects the pump beams at a small angle (e.g., less than 1 degree), the resulting signal beam is a time-reversed replica of the probe beam and propagates in the opposite direction of the probe beam. A beam splitter can be used to separate the signal beam from the path of the probe beam for signal detection. In a three-dimensional configuration, the pump beams may be in the same plane and the probe beam may be in a different plane. Hence, the generated signal beam will not retrace the probe beam and can be directly collected by a photodetector without using a beam splitter. This increases the signal strength of the received signal and improves the signal-to-noise ratio.
The above four-wave mixing detection can be used to form a spectroscopic analyzer by coupling a four-wave mixing optical module to an atomizer. This spectroscopic analyzer can be used to analyze gas-phase, liquid-phase, and solid-phase samples. The atomizer vaporizes an analyte to produce a vapor sample and the four-wave mixing optical module performs optical measurements of the vapor sample. Such a spectroscopic analyzer may be used in a range of applications, including trace-concentration analysis using gas-phase atomizers with sample cells, circular dichroism spectroscopy, capillary electrophoresis, and liquid chromatography in various fields such as biotechnology, environmental, material engineering and science, and basic scientific research.